To our minds, the first definition, provided by Shamir and Ewald (2014) is the most inclusive and accurate, given that it includes the different definitions the word “organoid” has been associated with. It avoids the specific restriction imposed by other definitions and includes organoids generated from induced or embryonic stem cells. Researchers indeed are able to generate organoids in laminin-rich gels from single cells of normal tissues or malignant tumors, or even cell lines, without necessarily starting from cells that express stem cell markers (Weaver and Bissell, 1999). This is especially relevant given that we still do not understand whether a stem cell can be defined independently of its niche (Schofield, 1978; Mesa et al., 2015). Niches are specialized microenvironments located within each tissue where stem cells reside. Niches exert a key influence over stem cell function and are defined as the sum of the cell–cell, cell–ECM, and cell–soluble factor interactions, in the context of the physical and geometric constraints that a cell may experience at a given time (Kaplan et al., 2007). The ability of the niche to determine
